The continuous increase in global energy demand is forcing society to search for environmentally clean, sustainable and renewable energy sources. See Schipper, S. Meyer, R. Howarth and R. Steiner, Energy Efficiency and Human Activity: Past Trends, Future Prospects (Cambridge University Press, Cambridge, 1997) and K. Zweibel, Harnessing Solar Power: The Photovoltaics Challenge (Plenum Press, New York, 1990). Several alternate sources of energy such as wind, solar, hydro and biomass have been explored over the last several decades. Among all these unconventional energy sources, solar energy has emerged as a most practical alternative to conventional fossil-fuel based energy sources. The Sun provides 32×1024 joules every year. See K. Zweibel, Harnessing Solar Power: The Photovoltaics Challenge (Plenum Press, New York, 1990). If even 0.01% of the Earth's surface was covered with 10% efficient solar cells, present energy needs would be fully satisfied. However, despite the continuously increasing interest in solar energy, the present solar cell technology is still not able to compete fully with the conventional fossil energy sources due to the high manufacturing costs.
In recent years, dye-sensitized solar cells (DSSCs) have received considerable attention as a cost-effective alternative to conventional solar cells. See B. O'Regan and M. Grätzel, “A low cost, high efficiency solar cell based on dye sensitized colloidal TiO2 films”, Nature, 353:737-739 (1991); M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphrybaker, E. Muller, P. Liska, N. Vlachopoulos and M. Grätzel, “Conversion of light to electricity by cis-x2 bis(2,2′-bipyridyl-4,4′-dicarboxylate)-ruthenium(II), (x=Cl—, Br—, I—, Cn—, and Scn-) on nanocrystalline TiO2 electrodes”, Journal of the American Chemical Society, 115:6382-6390 (1993); M. Grätzel, “Photoelectrochemical cells”, Nature, 414:338-344 (2001); B. A. Gregg, “Excitonic solar cells”, Journal of Physical Chemistry B, 107:4688-4698 (2003); and M. Grätzel, “Dye-sensitized solar cells”, Journal of Photochemistry and Photobiology, C 4:145-153 (2003). DSSCs operate on a process that is similar in many respects to photosynthesis, the process by which green plants generate chemical energy from sunlight. In particular, dye molecules absorb light in the visible region of the electromagnetic spectrum and then “inject” electrons into the nanostructured semiconductor electrode. See M. Grätzel, “Photoelectrochemical cells”, Nature, 414:338-344 (2001). This process is accompanied by a charge transfer to the dye from an electron donor mediator supplied by an electrolyte, resetting the cycle. DSSCs based on liquid electrolytes have reached efficiencies as high as 11% under AM 1.5 (1000 W m−2) solar illumination. See B. A. Gregg, “Excitonic solar cells”, Journal of Physical Chemistry B, 107:4688-4698 (2003) and M. Grätzel, “Dye-sensitized solar cells”, Journal of Photochemistry and Photobiology, C 4:145-153 (2003). However, unfortunately liquid electrolyte based DSSCs have much smaller life times compared to their inorganic counterparts. This problem arises mostly because of the fact that all the present DSSCs use liquid electrolytes. Liquid electrolyte evaporates and causes leakage in the cell thereby limiting their stability and life span.
A conventional solid state DSSC 100 is shown in FIG. 1. The DSSC includes substrates 102 each coated with a transparent conducting oxide (TCO) 104 and a mesoporous TiO2 network 106. The mesoporous TiO2 is coated with dye 108. The DSSC also includes a solid electrolyte 110, CuBO2. As shown in the figure, a problem with such solid state DSSCs is the difficulty in inserting the solid electrolyte inside the dye coated mesoporous TiO2 network. Because of the poor incorporation and dispersion of the solid electrolyte inside the TiO2 pores, the short circuit current is low, resulting in the low conversion efficiency of the cells.